In point-to-multipoint (PMP) communications, there is an inherent asymmetry in the tolerance of costs for hub and customer premise equipment (CPE). There is a desire to reduce the cost of CPE, even if it increases the cost of hub equipment.
Communication systems have encountered problems in the presence of high multipath and scattering, and high path loss due to obstructions, shadowing and absorbtion. Multipath is the phenomenon where signals, though transmitted from a single point can appear at the receive point as multiple delayed versions superimposed on themselves, where generally, the received signal energy appears to emanate from various spatial locations due to reflections and scattering of signals.
There are many causes for this behavior, called “multipath”. When there is a physical blockage in the line-of-sight path between the transmit antenna and the receive antenna, the only signals available at the receive antenna are those subjected to the multipath condition. Additionally, due to scattering of signals in the transmission path, signals can find multiple paths from the transmitter to the intended receiver. Finally, in such a transmission environment, signals transmitted from a source can find their way to non-intended receivers, thus interfering with their reception of desired signals. Path loss in free space is well understood. However, in non-line-of-sight environments, terrain, obstructions that shadow or absorb signals, and destructive interference from multipath can cause substantially higher path loss.
In PMP systems, the classical approach has been to use single carrier signals modulated with BPSK, QPSK N-ary PSK or QAM modulation formats. The classical approach to receiving such single carrier signals is to use a decision feedback equalizer. This device attempts to equalize the channel by estimating the channel and subtracting out the estimated multipath energy. Specifically, such an equalizer implements an algorithms similar to the following to estimate the transmitted signal, based on the received signal:T=(R−(A2Rz−1+A3Rz−2 _ _ _ AnRz−n))/A1
Where T=estimate of transmitted signal
R=received signal
A1-An=estimate of channel in the form of an FIR filter
z−n=delay element where n represents the number of delay states
This approach can lead to substantial performance degradation in noisy or, high multi-path environments. Also, the classical decision feedback equalizer requires a long time to converge on a good estimate of the channel in noisy and high multipath environments.
Later approaches involved utilizing spread spectrum techniques in which the data signal is spread by multiplying it with a higher rate spreading sequence, where delayed versions of the spreading sequences are highly uncorrelated to the original spreading sequence. Using this property, spread spectrum signals are received by using multiple correlators, called a Rake Receiver, wherein the signal and its multipath elements can be isolated and independently demodulated.
This technique is effective if the multipath delay spread (time delay between the earliest and latest received path for a given transmission) is less than 10% of the symbol time of the modulated data signal. This holds true for wireless systems transmitting low data rate signals, but not for ones transmitting high data rate signals. If the delay spread is significantly greater than 10% of the symbol time, then equalizers must be used—resulting in the same issues as equalizers for reception of single carrier transmissions.
Recently, companies have been advocating the use of multi-carrier modulation schemes such as orthogonal frequency division multiplexing (OFDM), orthogonal code division multiplexing (OCDM), or multicarrier CDMA to allow the receiver to better compensate for multipath and scattering encountered in the transmission channel. The basic concept is to divide the high data rate signal into multiple lower data rate signals and to transmit each low data rate signal using a different frequency tone (in the case of OFDM), or code (in the case of OCDM or multicarrier CDMA).
The techniques developed to receive these modulation formats allow them to work in high multipath environments with nominal degradation in performance. For example, a properly designed OFDM system may have its useable performance degraded by 2-4 dB in the presence of high multipath, where the same measure of performance in conventional equalizer-receivers using single carrier approaches may degrade by substantially larger amounts, if they are able to receive the signal at all.
However, the multi-carrier modulation schemes inherently have a high peak to average power ratio. This ratio requires increased linearity in non-linear components such as transmit upconverters and power amplifiers. This increased linearity is achieved by a combination of backoff (operating the amplifier at a lower power where it operates more linearly) and linearization (where non-linearities are compensated for through pre-distortion or feed-forward inter-modulation cancellation). Both these linearization techniques add a great deal of cost to the amplifier and therefore are considered undesirable in CPE.
Various modulation techniques have been developed to combat multipath. However, few techniques have been developed to overcome the higher path loss associated with non-line-of-sight transmissions.